Conventionally, Al alloys are used in various fields, for example manufacturing of automotive heat exchangers and automotive valve train because they are light weight, superior in corrosion resistance, have high thermal conductivity and other desirable properties.
Al alloy members are joined together, or an Al alloy member is joined to another member made of a different material, by various methods, including welding, brazing, internal chilling and soldering.
In the case of brazing, JIS;BA4045 of Al--Si group, JIS;BA4145 of Al--Si--Cu group, alloy brazing material of Al--Si--Mg group and others are used as the brazing material.
When such brazing materials are used, the components of the brazing material and the joined materials are mutually diffused, thereby new intermetallic compounds are formed or foreign substances are precipitated in the joining layer formed at the joining part, i.e. close to the surface of the joined materials.
Since such intermetallic compounds or precipitated substances exists in the joining layer, the joining layer has high yield strength. However, if the intermetallic compounds or precipitated substances are too concentrated, the joining layer becomes brittle because the yield strength is too high.
Especially if an Al alloy is joined with a member made of for example a ceramic material, W alloy or Fe group alloy, whose coefficient of thermal expansion is significantly different from that of the Al alloy, the residual stress generated after the joining is so large that the buffer effect in the joining layer cannot be achieved. As a result, a fracture may occur in the intermetallic compound phase.